In marine trunk piston engines, Heavy Fuel Oil (‘HFO’) is generally used for offshore running. Heavy Fuel Oil is the heaviest fraction of petroleum distillate and comprises a complex mixture of molecules including up to 15% of asphaltenes, which are defined as the fraction of petroleum distillate which is insoluble in an excess of aliphatic hydrocarbon (e.g. heptane) but which shows solubility in aromatic solvents (e.g. toluene). Asphaltenes can enter the engine lubricant as contaminants either via the cylinder or the fuel pumps and injectors, and asphaltene precipitation can then occur, manifested in ‘black paint’ or ‘black sludge’ in the engine. The presence of such carbonaceous deposits on a piston surface can act as an insulating layer, which can result in cracks forming, which then propagate through the piston. If a crack travels right the way through, then hot combustion gases can enter the crankcase, which may result in a crankcase explosion.
A key design feature of trunk piston engine oils (‘TPEO’s) is prevention of asphaltene precipitation but, with the current use of Group II base oils which have a lower aromatics content, their effectiveness in this respect has been reduced.
WO 96/26995 discloses the use of a hydrocarbyl-substituted phenol to reduce ‘black paint’ in a diesel engine. WO 96/26996 discloses the use of a demulsifier for water-in-oil emulsions, for example, a polyoxyalkylene polyol, to reduce ‘black paint’ in diesel engines.
The aim of the present invention is to reduce asphaltene precipitation or ‘black paint’ in an engine, in particular, a marine diesel engine. The aim of the present invention is also to reduce asphaltene precipitation or ‘black paint’ in an engine using a lubricating oil composition comprising a Group II base stock.